UNIT 3 : CHEMICAL FORMULA AND EQUATIONS

Relative Mass

RAM: To state the meaning of relative mass and solve numerical problems
FMM: to state the meaning of RAM,RMM and RFM based on carbon-12 scale

RMM: to calculate RAM, RMM and RFM using the chemical formulae of various substances

1. a single atom is too small and lightr and cannot be weighted directly.
2. The best way to determine the mass of a single atom is to compare it mass to the mass of another atom of an element that is used as a standard.
3. Ram has no unit. The new standard today is the carbon-12 atom, Ram based on the carbon-12 scale is the mass of one atom of the element compared with 1/12 of the mass of an atom of carbon-12.

4. RMM / RFM is obtained by adding up the RAM of all the atoms that are present in the molecule.

Molecular substance	Molecular Formula	Relative Molecular Mass
Oxygen	O2
Water	H2O
Carbon dioxide	CO2
Ammonia	NH3

5. For ionic substance, RMM is replace with relative formula mass(RFM)

Substance	Chemical Formula	Relative Molecular Mass
Sodium chloride	NaCl
Potassium oxide	K2O
Copper(II) sulphate	CuSO4
Ammonium carbonate	(NH4)2CO3
Aluminium nitrate	Al(NO3)3
Calcium hydroxide	Ca(OH)2
Lead(II)hydroxide	Pb(OH)2
Hydrated copper(II) sulphate	CuSO.5H2O

Mole concept

Mole and number of particles: - to solve numerical problems involving mole and the number of atoms

Mole and the mass of substances / : to solve numerical problems involving mole and the mass of

Mole and volume of gas substances, number of particles and volume of gas using mole concept.

1. To describe the amount of atoms, ions or molecules, mole is used.
2. A Mole is an amount of substance that contains as many particles as the number of atoms is exactly 12 g of carbon-12
3. Amol of a substance is the amount of substance which contains a constant number of particles( Atoms, Ions,Molecules), which is 6.02 x 10 ²³
4. The number of 6.02 x 10 ²³ is called the Avogadro constant(N_A)
5. For compunds that exist as molecule/ions, the number of atoms/ions in that compunds must be known
6. The symbol of mole is mol.
   Number of moles and mass of substance
   1. Molar Mass:
   a) Molar mass is the mass of one mole of any substance
   b) Molar mass is the relative atomic mass, relative molecular mass and relative formula mass of a substance in gmol^-1.
   c) Molar mass of any substance is numerically equal to its relative mass.

Number of moles and volume of gas.

1. Molar volume of gas : Volume occupied by one mole of any gas is 24dm₃ at room conditions and 22.4 dm₃at standard temperature and pressure(STP)
2. the molar volume of any gas is 24dm₃ at room conditions and 22.4 dm₃ at STP.
3. Conclusion: One mole of any gas always occupy the same volume under the same temperature and pressure.

Chemical formula and equations

Empirical formula : to state the meaning of empirical formula and describe the laboratory activities

to determined empirical formula

Molecular formula: to solve numerical problems involving empirical formula

Chemical Formulae: to write the formulae of common cations and anions, and to write the chemical formulae for ionic compounds

Chemical Equations : to write balanced chemical equations and solve numerical problems involving mole concept

1. Symbol of elements – use the first capital letter,
   if there is more htan one which share the same first capital letter thus it mus used together with small letters.
   Example:
   Carbon = C, Hydrogen = H
   Nitrogen= N, sodium = Na
   Sulphur = S, Silicon = Si

2. Chemical formula is a set of chemical symbols for atoms of elements in whole numbers representing chemical substances.

Chemical substance	Chemical formula	Notes
Water	H2O	2 atoms of H combine with 1 atom of O
Ammonia	NH3	3 atoms of H combine with 1 atom of N
Propane	C3H8	3 atoms of C combine with 8 atoms of H

3. Information that can be obtained from the chemical formula:
   a) all te element present in the compound
   b) Number of atoms of each element in the compound
   c) calculation of RMM / RFM of compound.
4. Empirical formula = a formula that shows the simplest whole number ratio of atoms of each element in the compound
5. Ehe formula can be determined by calculating the simplest ratio of moles of atoms of each element in the compound. EF can be obtained by experiment
   example : the empirical formula of Magnesium oxide or copper(II) oxide

6. To calculate the empirical formula of a compound, use the following table:

Element
Mass of element(g)
Number of moles of atom
Simplest ratio of moles

Molecular Formula of a compound shows the actual number of atoms of each element that are present in a molecule of the compound

Molecular formula = ( EF ) n, where n is integer ( the repeatation of the EF )

Compound	Molecular formula	Empirical formula	Value of n
Water	H2O	H2O	1
Carbon dioxide	CO2	CO2	1
Sulphuric acid	H2SO4
Ethene	C2H4	CH2	2
Benzene	C6H6	CH
Glucose	C6H12O6

7. Percentage composition by mass of an element in a compound

example: calculate the percentage composition by mass of nitrogen in the following compounds

[RAM: N=14, H=1, O=16, S=32, K=39]

i- (NH₄)₂SO₄ ii- KNO₃

% N = 2 x 14 x 100%        % N = 14 x 100%

           132                                101

       = 21.2%                           = 13.9%

8. Chemical formula of an ionic compound comprising of the ions X^m+ and Y^n- is by exchanging the charge on each ions. The formula obtained will be X_mY_n
   Example:
   Copper (ii) nitrate Zinc oxide
   Cu²⁺ NO₃^-   = Cu (NO₃)2 = Cu(NO₃)₂        Zn²⁺ O^{2-   =} Zn O = ZnO

Chemical equation

9. a) type of chemical equation-
   i.equation in words – using names of reactants and products
   ii. equation using symbols – represented by chemical formula and symbols
   examples:
   + seperating reactants / products
   (g) gaseous
   (aq) aqueous
   (s) solid

Sign / symbols	Meaning	Sign / symbols	Meaning
+	seperating reactants / products	∆	Heating/ heat energy is given
(g)	gaseous	→	Produces
(aq)	aqueous	↓	precipitate
(l)	liquid	↑	Gas release
(s)	solid		Reversible reaction